Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, small animals and the like on surfaces. According to some estimates, over 1,800 species comprising over 4,000 organisms are responsible for biofouling. Hence, biofouling is caused by a wide variety of organisms, and involves much more than an attachment of barnacles and seaweeds to surfaces. Biofouling is divided into micro fouling which includes biofilm formation and bacterial adhesion, and macro fouling which includes the attachment of larger organisms. Due to the distinct chemistry and biology that determine what prevents them from settling, organisms are also classified as being hard or soft. Hard fouling organisms include calcareous organisms such as barnacles, encrusting bryozoans, mollusks, polychaetes and other tube worms, and zebra mussels. Soft fouling organisms include non-calcareous organisms such as seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a fouling community.
In several situations, biofouling creates substantial problems. Biofouling can cause machinery to stop working, water inlets to get clogged, and heat exchangers to suffer from reduced performance. Hence, the topic of anti-fouling, i.e. the process of removing or preventing biofouling, is well-known. In industrial processes involving wetted surfaces, bio dispersants can be used to control biofouling. In less controlled environments, fouling organisms are killed or repelled with coatings using biocides, thermal treatments or pulses of energy. Nontoxic mechanical strategies that prevent organisms from attaching to a surface include choosing a material or coating for causing the surface to be slippery, or creating nanoscale surface topologies similar to the skin of sharks and dolphins which only offer poor anchor points.
Anti-fouling arrangements for cooling units that cool the water from a cooling water system of an engine-driven ship by means of seawater are known in the art. For example, DE 102008029464 relates to a box cooler for use in ships and on offshore platforms, comprising an integrated anti-fouling system for killing fouling organisms by means of an overheating process that can be regularly repeated. In particular, the box cooler is protected against microorganism fouling by continuously overheating a defined number of heat exchanger tubes without interrupting the cooling process, wherein waste heat from the cooling water may be used for doing so.
A box cooler is a specific type of heat exchanger which is designed for use in an engine-driven ship. For example, in the case of a tugboat having an installed engine power of 15 MW, one or more box coolers are applied for transferring heat in the order of 5 MW to the seawater. Typically, for the purpose of accommodating the tubes of a box cooler, a ship has a compartment that is defined by a portion of the hull of the ship and partition plates. Entry and exit openings are arranged in the hull at the position of the compartment so that seawater can enter the compartment, flow over the tubes in the compartment, and exit the compartment through natural flow and/or under the influence of motion of the ship. The box cooler comprises bundles of U-shaped tubes for conducting a fluid to be cooled, ends of leg portions of the tubes being secured to a common plate having openings for providing access to both leg portions of each of the tubes. The environment of a box cooler is ideally suited for biofouling, as the seawater is heated to a medium temperature in the vicinity of the tubes as a result of the heat exchange with the relatively hot fluid in the interior of the tubes, and the constant flow of water continuously brings in new nutrients and organisms.
Biofouling of box coolers causes severe problems. The main issue is a reduced heat transferring capability as layers of biofouling are effective heat isolators. When the biofouling layers are so thick that seawater can no longer circulate between adjacent tubes of the box cooler, an additional deteriorating effect on the heat transfer is obtained. Thus, biofouling of box coolers increases the risk of engine over-heating, so that ships need to slow down or ship engines get damaged.